1. Technical Field
The present invention relates to the manufacture of integrated circuits. More particularly, the present invention relates to the cooling of semiconductor wafers during the manufacture of integrated circuits.
2. Description of the Prior Art
Intense heat is often used during the various process steps that are necessary to fabricate integrated circuits on silicon wafers. It is then necessary to cool the wafers because the wafer temperature is usually in the range of 500.degree. C. to 800.degree. C. after such processing steps, making transport difficult (e.g. in a standard plastic wafer carrier), and limiting the ability to perform subsequent processing steps that may require lower wafer temperatures.
When the temperature of the wafer is in the 500.degree. C. to 800.degree. C. range, the wafer cools rapidly by radiation, i.e. by thermal transfer to the ambient. However, when the wafer has cooled such that the wafer temperature is in the 300.degree. C. range, the rate at which the wafer cools by radiation slows considerably. The wafer cooling rate slows because the rate of cooling by radiation is proportional to the wafer temperature raised to the fourth power. That is, the amount of decrease in the rate at which the wafer cools by radiation becomes greater as the wafer temperature decreases.
It is necessary to cool a processed wafer from the 300.degree. C. range to the 100.degree. C. range to allow the wafer to be loaded into a standard plastic wafer handling cassette for additional processing, testing, and/or assembly. Typically, such cooling proceeds in a cooling chamber under vacuum. Current industry practice involves the following steps to accomplish such wafer cooling:
1. The processed wafer is transferred to a water cooled pedestal in a cool down chamber under vacuum to minimize exposure of the wafer to the ambient, which would otherwise contaminate the wafer.
2. The chamber pressure is increased to several bar (Torr) by introducing an inert gas, such as argon, between the processed wafer and the water cooled pedestal. The inert gas provides a thermal transfer medium that conducts heat from the wafer to the water cooled pedestal. In most applications, the inert gas is applied to a backside of the wafer to minimize the amount of pressure within the chamber. In such arrangement, the wafer is retained in position relative to the pedestal by a mechanical clamp. PA1 1. Introducing an inert gas into the cooling chamber also introduces gas-borne particles into the chamber, which may contaminate the wafer and thus reduce the number of devices yielded by the wafer. PA1 2. Increasing chamber pressure picks up and circulates any sediment that may have collected at the bottom and sides of the chamber, thus increasing the likelihood of wafer contamination. PA1 3. The time required to increase chamber pressure to introduce an inert gas to assist in cooling the wafer and then pump down the chamber to remove such gas after the wafer has been cooled increases the amount of time necessary to cool the wafer and thus degrades processing throughput time. PA1 4. The wafer clamps used to hold the wafer in place during cooling must be precision manufactured and they thus add significant manufacturing cost to that of the cooling chamber. PA1 5. Electrostatic chucks, if used instead of wafer clamps, must include an electrically insulating layer on the entire wafer cooling surface of a pedestal between the chuck and the wafer. The insulating layer is made of ceramic materials, such as thermally conductive silicon, which are nonetheless poor thermal conductors. Thus, electrostatic chucks, while capable of holding a wafer to a water cooled pedestal, thereby obviating the need for a mechanical wafer clamp (but not eliminating the need for a thermal transfer medium, such as an inert gas), tend to inhibit heat transfer from the wafer to the water cooled pedestal and thus significantly slow the wafer cooling process. Additionally, if an electrostatic chuck is used without a thermal transfer medium, the electrostatic chuck must be very thin to achieve better thermal conductivity between the wafer and the water cooled pedetal. However, the thin ceramic insulating layer of the electrostatic chuck is easily destroyed by the high voltages used to charge the chuck and hold the wafer thereto. Thus, state of the art electrostatic chucks require frequent servicing, which adds to the down time of the cooling chamber and slows wafer processing throughput time.
3. Cooling proceeds until the wafer is cooled to about 100.degree. C. The chamber is then pumped down to avoid increasing the base pressure of the wafer processing system and the wafer is transferred from the cooling chamber.
It is also known to use an electrostatic chuck to hold the wafer to the water cooled pedestal during wafer cooling instead of a mechanical wafer clamp. An electrostatic chuck is formed integral with the surface of the water cooled pedestal. When an electrostatic chuck is in operation, a static charge is maintained on the surface of the water cooled pedestal which attracts and holds the wafer to the surface of the pedestal during wafer cooling. U.S. Pat. No. 4,184,188, Substrate Clamping Technique In IC Fabrication Processes, issued to Briglia on Jan. 15, 1980 shows an electrostatic chuck of the type known in the art.
The following disadvantages are known in connection with the above approaches to wafer cooling:
It would be useful to be able to rapidly cool a processed wafer in a vacuum chamber without contaminating the wafer.